The present invention relates to a glass that is required to have a high thermal expansion coefficient as a material of optical filers used for wavelength division multiplexing (WDM) in optical communication to allow the passage of only a certain predetermined wavelength region.
In general, optical filters can be classified into those for cutting and transmitting certain predetermined wavelengths and those for lowering light transmittance. Examples of the former filters are (a) bandpass filter for transmitting only certain predetermined wavelengths, (b) notch-pass filter for cutting only certain predetermined wavelengths, (c) low-pass filter for transmitting only wavelengths shorter than a particular wavelength, and (d) high-pass filter for transmitting only wavelengths longer than a particular wavelength. A typical example of the latter filters is ND filter.
In wavelength multiplexing optical communication, bandpass filter is used for combining lights having slightly different wavelengths or for selectively taking a particular wavelength light out of a light containing a plurality of wavelength components.
Narrow bandpass filter has been proposed along the development of WDM system and is referred to as “WDM optical filter”.
Japanese Patent Unexamined (Kokai) Publication 10-339825, corresponding to U.S. Pat. No. 5,859,717, discloses a multiplexing device having a multilayer film formed on a substrate.
Japanese Patent Unexamined (Kohyo) Publication 10-512975, corresponding to U.S. Pat. No. 5,719,989, also discloses a dielectric multilayer film bandpass filter.
In course of the development of high-precision WDM systems, it is required to narrow the transmission wavelength bandwidth of a WDM optical filter in order to conduct a high-density wavelength multiplexing optical communication. Narrowing the transmission wavelength bandwidth leads to a narrowed band center wavelength. Therefore, even the wavelength center deviation caused by a slight temperature fluctuation may interfere with the bandpass filter performance. Thus, it is required to prevent a WDM optical filter from having a refractive index fluctuation caused by the temperature fluctuation upon its use. In other words, it is required to make the wavelength temperature shift close to zero.
It is known that the temperature shift depends on thermal expansion coefficient of the glass and that of the dielectric multilayer film of a WDM optical filter.
Japanese Patent Unexamined (Kokai) Publication 2001-89184, corresponding to U.S. Pat. No. 6,465,105, discloses a glass substrate for use in WDM optical filter. Japanese Patent Unexamined (Kokai) Publication 2001-66425, corresponding to U.S. Pat. No. 6,461,733, discloses a glass for a light filter. The glasses of these publications may be brittle due to their glass compositions. Thus, it may be possible that the glass substrate having such composition is broken at its corner portion when the glass substrate is cut by a diamond cutter into a chip, thereby lowering its yield.
Japanese Patent Unexamined (Kokai) Publication 2001-48584, corresponding to U.S. Pat. No. 6,410,466, discloses glass ceramics (crystallized glasses) for a light filter. It is naturally necessary to provide a crystallization step by conducting a long-time heating treatment in the production of glass ceramics. Such step may increase the production cost. Furthermore, it may be difficult to spot or notice the final product (filter chip) due to its small size (e.g., 1 mm square) and its colorlessness and transparency.